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post #1 of 11 Old 08-02-2007 Thread Starter
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Assemble thru-hull & seacock...how?

I have a 1 1/2" ID Conbraco bronze mushroom-head (NPS thread) thru-hull; the length of the threaded shaft is about 3". I also have a Forespar/RC Marine (Marelon) seacock.

1. West Marine catalog (http://www.westmarine.com/webapp/wcs...&classNum=null) says threads on the seacock are NPS which should jive with the thru-hull. However, the Forespar website, if I'm reading this page right (http://www.forespar.com/onlineCatalo...ocks2007.shtml) suggests threads are NPT. What am I not getting right here? Could I have ended up with a seacock with BSP threads?

2. Dovetailing to above question, when I dry-fitted the thru-hull into the seacock, I can get the first 4 full-turn threads on, but then the fit gets increasingly difficult to turn by hand...to the point where I can't get the thru-hull to move at all. Do I have a thread mis-match here? (I'm thinking about exchanging the Marelon seacock for bronze...only about $12 more).

3. And this is the crux of my post. Our hull (1981 Oday 28) is about 1/2" solid fiberglass. The backer block I intend to install is 1/2" fiberglass which will be epoxied into position. This means I have 1" of "hull thickness" for the thru-hull's tube. But the depth of the seacock receiving end is about 1 1/2". I intend to have the thru-hull bottom out into the seacock and, thus, the seacock flange sitting flush with backer block. Then, I'll sink screws through seacock flange holes into backer block...don't want three more little holes in hull. Can I assume that I will have to cut off about 1/2" off of the threaded end of the thru hull (remember the shaft of this thru hull is about 3") so it can bottom out in the seacock (or get close) as the seacock fully engages to the backer block? Or should I plan to "shim" the backer block?

What is the preferred method for installing these parts? In advance, thanks for your input. Best regards.
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post #2 of 11 Old 08-03-2007
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First of all, I wouldn't mate a marelon fitting with a bronze, particularly one that will be below the waterline. You might also check and see if the thru-hull has tapered threads, which is why it will only go on so far.

If you're going to use a flanged seacock and a backing plate and have a 1/2" left over, it would seem easiest to just put another 1/2" piece of fiberglass there to make up the "leftover". Or use an unflanged seacock, especially if there's much curvature at that spot.

Just put one of each type of seacock on new thru-hulls on my boat, and made the choice by how rounded that part of the hull was.

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post #3 of 11 Old 08-03-2007 Thread Starter
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Dryfitted to extra ball valve and guess what?

I had an extra 1 1/2" Forespar Marelon Ball Valve lying around, so I tried dry-fitting thru-hull on that. Well, threaded all the way up and bottomed out. So, back to the seacock.
The top of this seacock is also supposed to be threaded NPT. So, dry fitted the thru-hull to the top of this seacock and voila! Threaded all the way up.
I am starting to suspect one of two things happened. One, someone bought this and threaded an NPS thru-hull (didn't think they made them) and screwed up (no pun intended) the threads of the seacock, and then returned it (now I've got it)...OR two, Forespar put the wrong "bottom" on this seacock.
In any event, I think I'll just return it for an Appollo bronze seacock (think I'll take the thru-hull with me and dry fit in the store).
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post #4 of 11 Old 08-03-2007
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I would agree that using a Marelon seacock over a bronze through-hull is probably not the best idea. The expansion and contraction of the two materials is probably going to cause problems in the long run... since the bronze probably expands far more than the Marelon, and the Marelon will be in tension around the bronze through-hull.

The bronze through-hull is NPS
, rather than NPT. The Marelon seacock is NPT, and as such will have some problems if used with the NPS-threaded through-hull. Either get a Bronze seacock that is NPS-threaded or get a Marelon through-hull that is NPT-threaded. Please note in the two attached photos that the Conbraco are clearly NPS-threaded, and that the Marelon Seacocks are definitely NPT-threaded.

You would probably want to cut the through hull to length to fit the seacock properly, so you can bolt the seacock down solidly. BTW, you will want to get some plumbing thread sealant. This is the kind I'd recommend: LINK




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Last edited by sailingdog; 08-03-2007 at 12:37 AM.
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post #5 of 11 Old 02-22-2008
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I would use Groco flange adapters. check out groco.net/new-prdcts-home/ibvf-08.htm
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post #6 of 11 Old 02-22-2008
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If you want a better understanding

If you want a better understanding read these:

Seacock & Thru-Hull Primer/Pre Information


Replacing Thru-Hulls and Seacocks


Basically you don't want to mix NPS and NPT which it sounds like you have. Forespar offers Marelon "in-line" valves as NPS but you definitely DON'T want NPS and NPS if the threads are the only thing holding that valve to the thru-hull!

SD is correct in that the mixing of bronze and Marelon is not advised.

The use of "in-line valves" screwed directly to a thru-hull without a proper flange is not the wisest choice!!

NPT vs. NPS



P.S. I like this quote by a fellow forum member:
"The problem I had was when I went to remove the frozen valve, it turned the thru hull, thus distroying the thru hull seal."

This should be reason enough to use properly flanged seacocks or as stated above flanged adapters from Groco.

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Last edited by Maine Sail; 02-22-2008 at 07:52 AM.
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post #7 of 11 Old 04-09-2008
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I have seen bronze through hull fittings that are NPS but are NPT just at the tip so they can fit an NPT ball valve fitting.
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post #8 of 11 Old 04-09-2008
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I seriously doubt that... since through-hulls are designed to be cut to the proper length, and you can't cut NPT threaded sections and expect them to fit anything properly.

Quote:
Originally Posted by string11 View Post
I have seen bronze through hull fittings that are NPS but are NPT just at the tip so they can fit an NPT ball valve fitting.

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post #9 of 11 Old 04-09-2008
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Must be commissioning season................. What is this the fourth Thru-Hull thread today??????

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post #10 of 11 Old 04-09-2008
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Well I am a machinist, and while I am not a thread expert, I can tell when a straight thread turns into a tapered thread. I don't know if it is the national standard taper and I don't know if the design intent was to make them compatible with a tapered female threaded fitting but these through hulls were definitely tapered at the tip.

Last edited by string11; 04-09-2008 at 04:49 PM.
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